1. Field of the Invention
This invention relates to liquid ink compositions containing colorant pigment particles treated with a polymer comprising units derived from at least a nitrogen-containing polymerizable monomer. In particular, this invention relates to liquid ink which exhibits improved chargeability, reduced conductivity variability, and improved dispersion stability when used in any imaging process, including but not limited to ink transfer processes, ionographic, electrographic, and electrophotographic color printing or proofing processes.
2. Background of the Art
Liquid inks are widely used in a variety of imaging and printing processes, for example offset, bubble jet, ink jet, intaglio, rotogravure, ink jet, electrographic and electrophotographic printing. Many of the desired characteristics of the pigment dispersions used in the liquid inks are the same for each of the respective processes even though the final ink formulations may be substantially different. For example, the stability of the pigment dispersion on the shelf, under shear conditions, and under high voltage fields is an important consideration regardless of the final use of the liquid ink. The art continuously searches for more stable pigment dispersions to provide more flexibility in ink formulations, which in turn yields better efficiency and waste reduction in the various printing processes.
In electrophotographic applications, which includes devices such as photocopiers, laser printers, facsimile machines and the like, liquid inks are referred to as liquid toners or developers. Generally, the electrophotographic process includes the steps of forming a latent electrostatic image on a charged photoconductor by exposing the charged photoconductor to radiation in an imagewise pattern, developing the image by contacting the photoconductor with a liquid developer, and finally transferring the image to a receptor. The final transfer step may be performed either directly or indirectly through an intermediate transport member. The developed image is usually subjected to heat and/or pressure to permanently fuse the image to the receptor.
Liquid toners typically comprise an electrically insulating liquid that serves as a carrier for a dispersion of charged particles known as toner particles composed of a colorant (e.g., pigment or dye) and a polymeric binder. A charge director is often included as a component of the liquid developer in order to regulate the polarity and magnitude of the charge on the toner particles. Liquid toners can be categorized into two primary classes. For convenience, the two classes will be referred to as conventional liquid toners and organosol toners. Among these two classes of liquid toners, organosol toners are preferred in electrophotographic applications for their stability.
Stable organosols contain colloidal (0.1-1 micron diameter) particles of polymeric binder. The particles are typically synthesized by nonaqueous dispersion polymerization in a low dielectric hydrocarbon solvent. These organosol particles are sterically-stabilized with respect to aggregation by the use of a physically-adsorbed or chemically-grafted soluble polymer. The most commonly used non-aqueous dispersion polymerization method is a free radical polymerization carried out when one or more ethylenically-unsaturated (typically acrylic or methacrylic) monomers, soluble in a hydrocarbon medium, are polymerized in the presence of a preformed amphipathic polymer. The preformed amphipathic polymer, commonly referred to as the stabilizer, is comprised of two distinct units, one essentially insoluble in the hydrocarbon medium, the other freely soluble. When the polymerization process to manufacture the organosol particle proceeds to a fractional conversion of monomer corresponding to a critical molecular weight, the solubility limit of the polymer is exceeded and the polymer precipitates from solution, forming a xe2x80x9ccorexe2x80x9d particle. The amphipathic polymer then either adsorbs onto or covalently bonds to the core, which core continues to grow as a discrete particle. The particles continue to grow until monomer is depleted, and the attached amphipathic polymer xe2x80x9cshellxe2x80x9d acts to sterically-stabilize the growing core particles with respect to aggregation. The resulting non-aqueous colloidal dispersion (organosol) comprises core/shell polymer particles with a number average diameter in the range 0.1-0.5 microns.
The resulting organosols can be subsequently converted to liquid toners by simple incorporation or mixing of the colorant pigment and a charge director, followed by high shear homogenization, ball-milling, attritor milling, high energy bead (sand) milling or other size reduction processes or mixing means known in the art for effecting particle size reduction in forming a dispersion. The input of mechanical energy to the dispersion during milling acts to break down pigment agglomerates into primary particles (0.05-1.0 micron diameter) and to xe2x80x9cshredxe2x80x9d the organosol into fragments that adhere to the newly-created pigment surface, thereby acting to sterically-stabilize the pigment particles with respect to aggregation. The result is a sterically-stabilized, charged, non-aqueous pigment dispersion having particles in the size range 0.1-2.0 microns number average diameter, with typical toner particle number average or weight average particle diameters between 0.1-0.5 microns. Such a sterically-stabilized dispersion is ideally suited for use in high resolution printing.
The charging of liquid inks is highly dependent upon the pigment used. The amount of charge director, also known as charge control agent, required for acceptable ink conductivity varies greatly among different colorant pigments and is designed into the ink according to known charge director design principles. Inks comprising different colorant pigments behave differently under printing conditions such as exposure to high electric fields. These variations, arising from the different charging characteristics of colorant pigments, are undesirable. There is also a concern about lot-to-lot variation of a colorant pigment, which might lead to undesirable variation in ink properties. Therefore, it is desirable to have ink conductivity and electrical properties independent of the type or lot of a colorant pigment. Furthermore, inks with lower levels of charge director, especially black ink that currently has a high level of charge director in the ink composition, is useful to decrease printed optical density and reduce overtoning problems such as tailing, washoff, and sludging (well known adverse influences in imaging processes). Previous attempts to increase the chargeability of inks (i.e. to prepare inks with sufficient conductivity at reduced charge director levels to provide effective imaging have either been ineffective to improve the chargeability or have resulted in undesirable effects such as high free phase conductivity. This term is defined in the section under toner conductivity and it is also a well know property in liquid electrophotograpy.
Lastly, most useful charge directors have a limited solubility in the low dielectric solvents suitable for electrophotographic liquid inks. As a result, the charge director aggregates as dispersed particles in the liquid or dispersed in the binder of the liquid ink. Consequently, there will be little direct physical contact or interaction between the colorant pigment and the charge director and ineffective ink transfer under electrostatically driven transfer processes.
There are some references known in the art to improve dispersion stability and charge characteristics of liquid inks. In U.S. Pat. No. 4,665,011, a liquid ink is described comprising a dispersed pigment and a block copolymer having a first block containing nitrogen-containing aromatic vinyl compounds and nitrogen-free aromatic vinyl compounds, an a second block containing polymerized C4-C6 dienes. The liquid ink is reported to have high dispersion stability and high charge stability.
In U.S. Pat. No. 5,009,980, an aromatic nitrogen-containing compound, which is substantially insoluble or immiscible in the non-polar liquid carrier, is dispersed in a thermoplastic binder as a charging adjuvant to form a liquid ink with improved image quality.
In U.S. Pat. No. 4,061,582, polyvinyl acetate is used as a binder as well as a coating on the surface of colorant pigment particles. The treated pigment particles require minimal miling to yield a stable liquid ink with improved oleophilic properties. However, there is no specific teaching of the use of nitrogen-containing polymer to surface-treated colorant pigments.
This invention provides a surface-treatment process consists of precipitating nitrogen-containing polymers in the presence of colorant pigments or of treating colorant pigments with a nitrogen-containing polymer solution and then drying the colorant pigments. The treated colorant pigments provide more highly charging liquid inks, inks with acceptable conductivity at a lower amount of charge control additive, and pigments with reduced lot to lot and manufacturer to manufacturer variation in charging properties.
In a first aspect, the invention features a process of making a liquid ink that includes (a) dissolving a polymer comprising units derived from at least a nitrogen-containing polymerizable monomer in a solvent with a Kauri-Butanol number greater than 30 to form a polymer solution; (b) dispersing colorant pigment particles in said polymer solution to form a colorant pigment dispersion; (c) removing at least some of the solvent from the colorant pigment dispersion to form treated colorant pigment particles; and (d) dispersing the treated colorant pigment particles in an organosol containing a carrier liquid having a Kauri-Butanol number less than 30.
In a second aspect, the invention features a process of making a liquid ink that includes (a) dissolving a polymer comprising units derived from at least a nitrogen-containing polymerizable monomer in a solvent with a Kauri-Butanol number greater than 30 to form a polymer solution; (b) dispersing colorant pigment particles in the polymer solution to form a colorant pigment dispersion; (c) precipitating treated colorant pigment particles from the colorant pigment dispersion; and (d) dispersing the treated colorant pigment particles in an organosol containing a carrier liquid having a Kauri-Butanol number less than 30.
In a third aspect, the invention features a liquid ink that includes (a) a carrier liquid having a Kauri-Butanol number less than 30; (b) an organosol; and (c) colorant pigment particles surface treated by a polymer comprising units derived from at least a nitrogen-containing polymerizable monomer.
The liquid inks of the present invention will be described with respect to electrophotographic office printing; however, it is to be understood that these liquid toners are not limited in their utility and may also be employed in printing or other ink transfer processes, such as high speed printing presses, photocopying apparatus, microfilm reproduction devices, facsimile printing, ink jet printer, instrument recording devices, and the like.